In studies designed to investigate the possibility that genes controlling colony specificity in Botryllus, a colonial tunicate, are related to those of the vertebrate major histocompatibility complex, we have determined that fusibility in this species is controlled by a single highly polymorphic gene locus, and are examining the relationship between genes controlling embryonic development and histocompatibility in Botryllus. In order to isolate and characterize humoral or cellular recognition elements which control allorecognition in these protochordates, we are producing antiserum and monospecific hybridoma antibody reagents from mice immunized with Botryllus cells, and are developing binding assays and functional bioassays for screening them for activity against fusion or rejection-related cell surface molecules.